Using relative URLs for content types across site collections is not supported

In Microsoft SharePoint Online, when you specify the Document Template for a site content type by using a URL of an existing document template, make sure that you use the URL of an existing site in the same site collection. A relative URL to a content type is not supported in the content type syndication scenario.

Specifically, under Site Content Type > Advanced Settings > Document Template, the URL (if any) has to be an URL of an existing content type on the same site collection.

Enter the URL for Document Template in Site Content Type: Advanced Settings page